The present invention relates to a gas leakage detection apparatus using a mass spectrometer.
The leakage detector commonly used is the helium detector, which essentially comprises a mass spectrometer adapted to detect the presence of helium. The analyser tube of the mass spectrometer is kept in a system under vacuum into which air containing helium is passed. As the concentration of helium in the air is low, the total pressure in the system under vacuum is given by the flow rate of the air. The partial pressure of the helium P.sub.He is proportional to the flow rate Q according to the equation: EQU Q=P.sub.He .multidot.S
wherein S is the pumping rate of the apparatus.
In practice, the sensitivity of detection is limited by two factors:
(1) the low sensitivity of the mass spectrometer in the range of very high vacuums where the noise level is very low: this is, in fact, the minimum detectable partial pressure which at present corresponds to 10.sup.-13 to 10.sup.-14 mm Hg of nitrogen (i.e. 1.333.times.10.sup.-11 to 1.333.times.10.sup.-10 Pa);
(2) the noise level which is much higher than the minimum detectable partial pressure when the operational pressure is maintained within the usual range of from 10.sup.-4 to 10.sup.-6 mm Hg (1.333.times.10.sup.-2 to 1.333.times.10.sup.-4 Pa)
The limitation of sensitivity due to the first factor can be overcome by increasing the flow rate in the system under vacuum and/or by throttling the pumping rate. These two actions have the effect of increasing the partial pressure of the gas, but they also result in an increase in the operational pressure of the system under vacuum and consequently in the noise level, which increases more or less proportionally with the total pressure. The limitation due to the second factor mentioned above makes it possible to detect a partial pressure five or six order of magnitude lower than the total pressure; for the range of pressures indicated above, the detectable partial pressure may be of the order of 10.sup.-9 to 10.sup.-12 mm Hg (1.333.times.10.sup.-7 to 1.333.times.10.sup.-10 Pa).
Various attempts have been made to reduce this dependence of sensitivity with regard to noise (for example, the use of a double concentration, baffle plates, ion counting) but all these attempts have resulted in fairly expensive equipment and the best results obtained have been the detection of partial pressures about eight orders of magnitude lower than the total pressure.